Online content sharing services are a popular mechanism for users to find and share content. For example, certain sites share content such as business recommendations (e.g., Yelp, TripAdvisor), news articles (e.g., Digg, reddit), multimedia content (e.g., Flickr, YouTube), applications (apps) (e.g., iOS App Store, Google Play), and uniform resource locators (URLs) (e.g., StumbleUpon, del.icio.us). Generally, these sites allow users to create accounts, declare friendships, and upload and rate content. Unfortunately, the increasing popularity of online content sharing sites has made them an attractive target for manipulation. They are also experiencing an increasing level of malicious activity including multiple identity (Sybil) attacks. Manipulation of ratings can be undesirable for a site operator, whose reputation is negatively impacted by successful manipulation, including the “buying” of ratings from users, as well as honest end users, who depend on the site to locate relevant and trustworthy content.